1. Field of the Invention
This invention relates to exhaust gas aftertreatment systems and more particularly to an apparatus and system for enhancing aftertreatment regeneration.
2. Description of the Related Art
Environmental concerns motivate emissions requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC).
The need to comply with emissions requirements encourages the development of exhaust gas aftertreatment systems. Aftertreatment systems frequently include one or more of a diesel oxidation catalyst (DOC), a NOx adsorption catalyst (NAC), and a diesel particulate filter (DPF). The DOC oxidizes unburned hydrocarbons in the exhaust stream for cleanup and/or temperature generation. The NAC adsorbs NOx from the exhaust gas and regenerates with periodic temperature events within the NAC. The DPF removes particulates from the exhaust gas stream. Furthermore, an exhaust gas recirculation (EGR) system may be implemented to reduce the formation of NOx during combustion.
Many aftertreatment components require temperature and/or UHC in the exhaust stream to facilitate regeneration, and many aftertreatment systems place a fuel injector (or “doser”) in the exhaust stream to provide the temperature and/or UHC. The placement of the fuel injector is a challenge in aftertreatment system design. In one embodiment of the present technology, the fuel injector is placed downstream of an exhaust manifold and turbocharger. Placement of the fuel injector, a precise mechanical device with sensitive electronic components, downstream of the exhaust manifold helps to ensure that commercially reasonable fuel injectors requiring relatively low operating temperature environments may be utilized.
A common alternative method for dosing the exhaust gas is “in-cylinder dosing.” The dosing fuel is injected directly into the combustion chamber ensuring that the fuel is thoroughly mixed with the exhaust before reaching the aftertreatment system. However, some of the challenges of in-cylinder dosing include diluting the engine oil with fuel, fuel recycling through the EGR, and the necessity of including a post-injection capable fuel system that may be more expensive than desired (e.g. a common rail fuel system).
Even if the fuel injector temperature limitations are overcome—perhaps through exotic materials and expensive cooling packages—placing the fuel injector into the exhaust manifold, or injecting in-cylinder, is difficult on engines with EGR. Fuel injected can be recirculated through the EGR path, potentially fouling an EGR cooler and EGR valve, and disrupting the designed torque and operation of the engine. Some engines may include grid heaters or other components in the air intake that are exposed to EGR flow and should not be exposed to unburned fuel. In the current technology, placing of a fuel injector in the exhaust manifold or dosing in-cylinder typically involves shutting off EGR and/or bypassing the EGR cooler. This results in increased emissions and/or lower power density of the engine.
Placement of the aftertreatment fuel injector downstream of the turbocharger presently causes performance limitations on the aftertreatment system. The placement downstream of the turbocharger means the fuel is injected into a cooler, low shear and low turbulence environment, closer to the component of interest—usually the DOC—and therefore the fuel may not be completely evaporated and distributed in the exhaust stream. Also, in the environment downstream of the turbocharger, the fuel does not experience enough time at temperature to begin breaking down from large hydrocarbon chains to small hydrocarbon chains, further reducing the oxidizing effectiveness of the DOC or other aftertreatment component.
An alternate placement of the aftertreatment fuel injector upstream of the turbocharger may allow for more flexibility of engine and aftertreatment design and permit fuel in the exhaust stream to experience higher temperatures, more turbulence, more shear forces, and longer residence time leading to superior oxidation and superior performance of the aftertreatment system.